A free window toolkit for Unity:
1.You can put the window prefab as a child gameobject of any UGUI gameobject.
2.You can move it by draging the top bar.
3.You can resize it by draging the borders or the corner points of the window
No special class or .dll inside, just basic Unity3D codes. It is proved that this project can run on from Unity 2017 to Unity 2018.